Radio receiver



C. J. BOERS RADIO RECEIVER Feb. 17, 1942.

Filed March 5, 1940 INVENTOR. coma/s J. BOERS- 776% QM ATTORNEY.

Patented Feb. 1 7, 1942 RADIQ RECEIVER Cornelis Jacobus Boers,Eindhoven, Netherlands, assignor to Radio Corporation of America, a

corporation of Delaware Application March 5, 1940, Serial No. 322,296 Inthe Netherlands April 11, 1939 3 Claims.

This invention relates to a wireless receiver provided with automatictuning correction and automatic gain control, the time constant of theautomatic tuning correction being greater than the time constant of theautomatic gain control.

A wireless receiver of this kind can be used conveniently with shortwave reception. Short Waves are subject to fading as are also the wavessituated in the medium-wave band. The fading occurring with short wavesis, however, differentiated from the fading in the medium-wave band bythe fact that the receiving intensity with short wavesmayvaryveryrapidly, say 25 times and more per second. In addition, withshortwave reception so-called selective fading may occur. The phraseselective fading is to be understood to define a fading which onlyoccurs in a narrow frequency band at one time. If the amplification ofthe receiver has to be controlled automatically in synchronism with thefading the time constant of the automatic gain control must be chosen tobe so small that even with very rapid fading sufiiciently rapid controloccurs. This time constant may be about /2 sec.

Selective fading presents difficulties when the carrier wave to bereceived is just comprised in the narrow frequency band in whichselective fading occurs, since in this case the carrier wave is receivedwith greater weakness compared with the side bands so that the chance ofover-modulation is very great. To remove or reduce this defect in thereceiver it is known to amplify the carrier wave to a greater extentthan the side bands. For this purpose in the receiver the frequency bandto be detected is fed to a sharp bandpass filter, for example a crystal,the side bands being thus suppressed to a greater extent than thecarrier wave. 1 In addition, provision must be made for the receiverfrequency band to be located relatively to the pass-range of the crystalso that the carrier-wave frequency lies in the middle of the said range,since the crystal is'so sharp that in the case of small divergences fromthe correct frequency the amplitude of the carrier wave decreases tosuch an extent that overmodulation occurs nevertheless. Such a receivershould, therefore, be provided with automatic tuning correction. In asystemof automatic tuning correction involving the use of. filtershaving passranges on either side of the frequency constancy of which isto be maintained, these filters, in order that they may be capable ofoperating efficiently, must be as sharp as the filter by which thecarrier wave is filtered. In this form of automatic tuning correctionthe receiver becomes, therefore, comparatively expensive.

Hence, use is preferably made of a system of automatic tuning correctionin which fewer filters are required, and, for example, of a system basedon phase differences between-the carrier wave present in the modulatedsignal and the carrier wave filtered from this signal which depend onthe tuning of-the receiver. This is done because in thiscase the sharpfilter already present can be made use of. In such a circuitarrangement, depending on phase difierences, for automatic tuningcorrection overmodulation, and the consequent reversal to the extent ofof the phase of the modulated carrier wave, is, however, responsible forthe control voltage which is supplied upon incorrect tuning changing itspolarity periodically. Thus, the disadvantage arises that the receiveris adjusted still farther away from the correcttuningpoint during thesemoments and this occurs very rapidly, because the control voltageinstead of decreasing as in the case of normal tuning correctionincreases during detuning.

For the purpose of eliminating the said difliculty, that part of thecircuit that governs the time which elapses between the variation of thefrequency, constancy of which is to be maintained, and the correction ofthis frequency may be proportioned so as to be possessed of a timeconstant of at least two seconds whereby the chance of any change of thepolarity of the control voltage is very small.

When fading occurs in these circumstances the amplification of thereceiver is controlled in synchronism. The anode current of thecontrolled tubes thus varies in .s ynchronism'with the fading and thisis responsible for the supply voltage varying in an identical manner .atthe terminals of the direct current supply source of the receiver. Thisvarying supply voltage is responsible in turn for a frequency modulationin synchronism with the fading of the frequency set up by one or moreoscillators present in the receiver. The carrier wave to be filtered bythe crystal is thus displaced in the same rhythm relatively to thepass-range of the crystal. In the case of rapid fading this gaves riseto difiiculties since the rapid displacement cannot be compensated bythe automatic tuning correction, because the latter has too great a timeconstant for this for the reasons set out above.

The invention has for its object to retard such rapid frequencymodulation, and according to the invention this is insured by the supplyvo1tages, which upon alteration lead to variation in frequency of one ormore local oscillators, being stabilized by means of a quick-actingstabilizing device.

Stabilizing of the supply voltages is preferably effected by means ofone or more gas discharge tubes. It would, also, be possible to maintainsufficient constancy of the supply voltages by choosing the timeconstant of the smoothing filter of the direct current supply source tobe much greater than the time constant of the automatic tuningcorrection. This solution is, however, costly, and requires much space.In a system of automatic tuning correction a change in frequency of oneor more local oscillators is brought about by means of a control voltagewhich is derived from the divergence from a frequency of the requisitevalue. This may be effected conveniently by the gain of a dischargetube, connected as a reactance in parallel with an oscillator circuit,being determined by means of the said control voltage, and the value ofthe said reactance being a function of the gain of the said tube. Thegain of this tube is, however, also dependent on one or more supplyvoltages fed to it, and for these reasons the supply voltages arestabilized according to a further feature of the invention.

In addition, variations in the supply voltages fed to one or moreoscillator valves will generally bring about variations in frequency sothat it is advisable also to stabilize these supply voltages.

In order that the invention may be clearly understood and readilycarried into effect it, will now be described more fully with referenceto the accompanying drawing. This drawing shows a diagram of connectionsof a wireless receiver, constructed according to the invention, providedwith automatic tuning correction and automatic gain control, the timeconstant of the automatic tuning correction being greater than the timeconstant of the automatic gain control. The signal frequency received bymeans of an antenna I is fed to a mixing valve 2 which is coupled to aseparate oscillator tube 3. The intermediate frequency set up in themixing valve 2 is chosen to be high in view of image frequencies thatmay occur, and is fed to a second mixing valve 6 with the interpositionof a bandpass filter 4 and an oscillatory circuit 5. The mixing valve 6converts the high intermediate frequency into a low one. The oscillatorycircuit 5 is tuned to the image frequency of the first intermediatefrequency relatively to the second oscillator frequency. The two valve 6which are nearest to the cathode act respectively as the control gridand the anode of the second local oscillator which comprises anoscillator circuit 9.

The oscillator circuit 5 has connected in parallel therewith thecathode-anode impedance of the discharge tube Ill; the control grid llof which has fed to it, with the interposition of a phase shiftingnetwork constituted by condensers l2 and I3 and a resistance l4, voltageobtained from the voltage occurring across the oscillatory circuit 9 butabout 90 out of phase therewith. The anode current of the tube H] is inphase with the grid voltage, and consequently also about 90 out of phasewith the anode voltage.

Thus, the tube [0 behaves as a reactance which is connected in parallelwith the oscillator circuit 9, and whose value depends on the gain ofthe tube 10.

The gain is governed not only by the value of grids I and 8 of themixing the supply voltages of the tube ill, but also by the biassupplied to the control grid II. The variation of the amplification oftube It is, therefore, responsible for a variation of the oscillatorfrequency, and thus of the intermediate frequency produced by the mixingvalve 8. This intermediate frequency is low, and. with the interpositionof an oscillatory circuit l5 tuned to this frequency and included in theanode circuit of the mixing valve 6, is fed to an inductance coil I6grounded at the middle point, to an oscillatory circuit Ill, and to oneof the diode anodes 18 of a duo-diode l9.

The inductance coil I6, supplies voltage to a crystal 2!] and, inphase-opposition, an equally high voltage to a condenser 2| which servesto neutralize the natural capacity of the crystal. Two resistances 22and 23 in parallel with the halves of the coil l6 and a resistance 24,between the connecting point of the crystal and the condenser 2i, serveto lessen the damping which the crystal experiences from the circuit Hi.The other terminal of the crystal 20 is connected to the control grid ofan amplifier valve 25 and, with the interposition of an inductance coil26, to ground. The anode circuit of the amplifier valve 25 includes aphase shifting network constituted by an inductance coil 21 in serieswith a variable resistance 28 which is shunted by a condenser 23. Theanode of the amplifier 25 is connected to the midpoint of the inductancecoil 30 of the oscillatory circuit H with the interposition of acondenser.

The ends of the oscillatory circuit ii are connected to the anodes oftwo diodes enclosed within a single tube envelope 3|. The cathodes ofthese diodes are connected by the series combination of two resistances32 and 33, the connecting point of which is connected to the'middlepoint of the coil 33. One of these cathodes is grounded. The othercathode is grounded with the interposition of a condenser 34, andconnected to the control grid ll of the discharge tube ill by aconductor and a resistance 14. Upon attainment of entirely correcttuning the phase relation of the amplified carrier wave and themodulated carrier wave which is transmitted to the circuit H byinduction is such that there is no potential difference between the twocathodes of the diode 3|. If, however, the intermediate frequency in thecircuit I5 varies, a positive or a negative control voltage is set upwhich brings about such variation of the reactance formed by thedischarge tube Ii] that tuning is corrected automatically. The timeconstant of the automatic tuning correction is substantially governed bythe values of the condenser 34 and of the resistances 32 and 33 andpreferably amounts to at least 2 seconds. The lead 35 is designated AFC.

In the diode formed by the diode anode i8 and the cathode of the valvei9 detection of the voltage across th circuit l5 occurs. The anodecurrent of the valve i9 is controlled by means of the unidirectionalvoltage obtained. The cathode of this valve is connected by a resistance5! to a point 3'! of negative potential relative to ground.

The valve [9 includes a further diode anode 36 which, with theinterposition of a series combination of two resistances 38 and 39, isconnected to a point ii] of lower negative potential relative to groundthan the point 31. So long as the grid voltage of the valve I9 is zerothe cathode is more positive than the diode anode 36, since a highervoltage drop occurs across the resistance 4!. When detection of thevoltage across the circuit occurs the anode current of the valve l9decreases, and the cathode falls in potential until the path of thediode anode 36 becomes conductive. At this moment the negative voltageof the diode anode 36 which was constant hitherto starts to become morenegativ'el -With the interposition of a filter constituted by acondenser 42 and a resistance 43, the voltage of the diode anode is fedto one or more preceding valves (in the drawing to the valve 6 only) forthe purpose of controlling the amplification. This is the automatic gaincontrol (A. G. C.) circuit. Th point of connection between theresistances 38 and 39 is connected by a similar R-C filter to aconductor 44 which feeds control voltage to a tuning indicator not shownin detail on the drawing. The time constant of the automatic gaincontrol which is governed by the resistances and condensers included inthe circuit is low in View of rapid fading, and amounts, for example, tosec.

The oscillations received from the crystal 20' which are constituted byan intermediate frequency carrier wave with side bands becoming feeblertowards the high modulation frequencies are transmitted to theoscillatory circuit 45 by the coil 26, after which detection occursbetween a diode anode 45 and the cathode of an amplifier valve 41, inwhich a diode is incorporated. The detected oscillations are fed to thecontrol grid 48 of the amplifier valve 41, said control grid beingconnected to ground with the interposition of the parallel combinationof a resistance 49 and a series resonance circuit constituted by aninductance coil 50 and a condenser 5| and tuned to the carrier wavefrequency. Thus an efficient by-pass is obtained for the carrier wavefrequency. 7

The anode circuit of the anode valve 41 includes a transformer 52 havinga low primary impedance so that the anode current of the amplifier valve41 is not affected thereby as regards its value. The voltage at theterminals of the secondary of the transformer consequently increaseswith increasing frequency so that the loss of high modulationfrequencies in the crystal 2i] is compensated. A final amplifier valve53 connected to the transformer 52 amplifies the low frequencyoscillations, and feeds them to a loudspeaker 54.

The receiver comprises a gas-discharge tube 55 which, in series with aresistance 56, is connected between ground and the positive terminal ofthe direct current supply source. When the voltage of this sourcefluctuates the voltage across the gas discharge tube 55 stays constant.This constant voltage is supplied to the screening grid' of thedischarge tube I0,'and to the oscillator anode 8 of the mixing valve 6with the interposition of a low pass filter constituted by twocondensers 51, 58 and a resistance 59. The said two electrodesconsequently remain at a constant potential and do not lead to variationof the frequency generated. In parallel with the gas discharge tube 55the resistances 5! and 62 are connected in series. The resistance 6! isa potentiometer, and the sliding contact thereof is connected to thecathode of the discharge tube It]. The current passing through theresistance BI is maintained con-- stant by the constant voltage of thegas discharge tube 55 so that the fixed grid bias of the tube l0 doesnot vary either. A further gas discharge tube 33 together with aresistance 64 provides in a similar manner for constancy of thesupplyvoltage of the first oscillator 3 to be maintained.

What is claimed is:

1.'In a superheterodyne' receiver of the type comprising a signal inputcircuit, a local oscillater provided with a tuned tank circuit and meansfor producing intermediate frequency energy from signals and localoscillations, a frequency control tube having at least a cathode,controlelectrode and anode, means to connect the anode to cathodeimpedance of the tube across'the tank circuit, means to apply to saidcontrol electrode alternating voltage in phase quadrature with the tankcircuit voltage, and means responsive to departures in frequency of theintermediate frequency energy for controlling the gain of said controltube; the improvement which comprises a source of energizing directcurrent voltage for the oscillator and control tube, energizingconnections from said source to the electrodes of the oscillator andcontrol tube, an automatic gain control circuit arranged to regulate thereceiver output in a sense to compensate for carrier fading, said gaincontrol causing frequency modulation effects of said oscillator duringrapid fading periods, means to supplement the frequency control of saidtank circuit during such periods, said last means comprising aquick-acting stabilizing device electrically connected with said voltagesource to maintain said energizing voltages applied to the electrodes ofthe oscillator and control tube substantially constant.

2. In a superheterodyne receiver of the type comprising a converter tubehaving a cathode, an oscillator control electrode, an oscillator anodeelectrode, a signal input electrode and an output electrode, a localoscillator tank circuit coupling the oscillator anode and controlelectrodes, an intermediate frequency output circuit coupled to saidoutput electrode, an electron discharge tube having its electrodesconstructed and arranged to provide a simulated frequency correctionreactance across the tank circuit, a relatively slow-acting meansresponsive to frequency deviation of the intermediate energy from apredetermined frequency value for controlling the gain of the correctiontube, a relatively fast-acting means responsive to signal carrieramplitude variations for varying the receiver gain in a sense toovercome the amplitude variations, said last means being connected tosaid signal input electrode and a source of direct current voltageconnected to the electrodes of said converter tube and correction tube;the improvement which includes means to supplement the frequencycorrection action of said slow-acting means during periods of relativelyfast variation of said carrier amplitude, said means comprising arelatively fast-acting voltage stabilizing device operatively associatedwith said voltage source, and a connection from the stabilizing deviceto an electrode of said converter tube to prevent frequency modulationeffects otherwise caused by the gain control means during said periods.

3. In a superheterodyne receiver of the type comprising a converter tubehaving a cathode, an oscillator control electrode, an oscillator anodeelectrode, a signal input electrode and an output electrode, a localoscillator tank circuit coupling the oscillator anode and controlelectrodes, an intermediate frequency output circuit coupled to saidoutput electrode, an elecjtron discharge tube having its electrodesconimprovement which includes means to supplement the frequencycorrection action of said slow-acting means during periods of relativelyfast variation of said carrier amplitude, said means comprising arelatively fast-acting voltage stabilizing device electrically connectedwith said voltage source, and a connection from the stabilizing deviceto an electrode of said converter tube and an electrode of thecorrection tube to prevent frequency modulation effects otherwise causedby the gain control means during said periods.

CORNELIS JACOBUS BOERS.

